The Mn--Zn system ferrite is used as a magnetic core for transformers and noise filters since it has the high magnetic permeability.
A pulse transformer used in S/T point interface of ISDN is sometimes installed in the open air when used for public telephones and digital service units (DSU). Therefore, the pulse transformer is required to have temperature compensation in the range from low to high temperature.
However, in case of the conventional Mn--Zn system ferrite, the initial magnetic permeability remarkably decreases in the low temperature range (-20 to 20.degree. C.). For this reason, in order to maintain the inductance at the low temperature range, it was necessary to increase a number of winding or to use a ferrite which is higher than necessary in the initial magnetic permeability at a normal temperature so that the initial magnetic permeability increases at the low temperature range. Hence, such a ferrite was not efficient at all.